Diverse types of petroleum feedstocks contain sulfur compounds whose removal is an indispensable prerequisite for commercial utilization of the feedstock, for subsequent processing of the feedstock, or both. Consequently, it is no surprise that substantial efforts have been expended to eliminate sulfur-containing materials from petroleum products. For example, the Claus process is commercially employed in removing hydrogen sulfide from feedstocks, at least for large streams containing large amounts (greater than about 1000 ppm) hydrogen sulfide. The Stretford process is a vanadium-based oxidative conversion of hydrogen sulfide to sulfur. A non-oxidative method of hydrogen sulfide removal is exemplified by the work of Bricker and Imai, U.S. Pat. No. 5,034,118.
Various oxidative processes also are known for removal of mercaptans by converting them to disulfides; many of these are available as the Merox.TM. process (see Handbook of Petroleum Refining Processes, R. A. Meyers, editor-in-chief, chapter 9.1, McGraw-Hill Book Company (1986)). It is also known to remove mercaptans and disulfides from petroleum feedstocks by adsorption with clays; see U.S. Pat. No. 5,360,536. In fact, adsorptive processes for sulfur removal may have elements of generality not shared by some oxidative processes.
U.S. Pat. No. 3,051,646 uses molecular sieves to selectively remove sulfur and sulfur-containing compounds such as mercaptans and disulfides. By using molecular sieve adsorbents with an average pore diameter of 8-20 angstroms the patentee avoided significant removal of hydrocarbon components. Hydrogen subsequently was used to desorb the adsorbed sulfur compounds and thus regenerate the molecular sieve. U.S. Pat. No. 3,211,644 teaches the use of crystalline zeolitic molecular sieve materials with an approximate pore size of at least 3.8 .ANG., including zeolite X, to adsorb sulfur-containing compounds from liquid hydrocarbon feedstocks with subsequent desorbtion of the sulfur compounds from the molecular sieves using a non-adsorbable purge gas, e.g., methane, hydrogen, nitrogen, and carbon dioxide. The patentee of U.S. Pat. No. 3,620,969 teaches that zeolitic molecular sieves dehydrated to a stated residual water loading may be used as an adsorbent for liquid hydrocarbon feeds with thermal swing desorption of the adsorbed sulfur compounds using a conventional purge gas with a high water content. U.S. Pat. No. 5,114,689 recognized the disadvantages and problems associated with the regeneration of molecular sieve adsorbents used in the desulfurization of hydrocarbon streams and suggested solutions thereto.
Although the prior art relating to the use of molecular sieves as adsorbents for removal of sulfur compounds is relatively well developed, there are unique problems in attempting to utilize molecular sieves for the analogous purification of FCC feedstocks. In particular, the different nature of FCC streams insures a substantially different nature in the sulfur-containing organic material. In fact, the sulfur-containing organic material in FCC streams are highly aromatic, in contrast to the sulfur streams of, for example, distillate gasoline, which has two important consequences. One consequence is that the nature of some major sulfur-containing organic materials is quite analogous to the major components of the FCC hydrocarbon matrix, making it more difficult to find molecular sieves which will selectively adsorb the offending sulfur-containing materials. A second consequence is that the adsorbed sulfur-containing material is sufficiently strongly adsorbed that regeneration of the molecular sieve by conventional means is ineffective. Since once-through use of adsorbents simply is impractical and quite uneconomical, regenerability of molecular sieves is a sine qua non for any commercially viable process.
We have devised an effective process to remove sulfur-containing compounds from FCC feedstocks based on certain molecular sieves impregnated with active hydrogenation metals. In particular, where the adsorbent is a potassium-exchanged zeolite Y with palladium or platinum dispersed thereon, we have found it is not only possible to selectively adsorb heterocyclic sulfur-containing compounds so prevalent in the FCC feedstocks without a concomitant significant loss of aromatic hydrocarbons, but it is also possible to effectively regenerate the sulfur-laden adsorbents. Regeneration is performed in a hydrogen atmosphere at elevated temperatures in what is in effect a reductive desulfurization stage.